The present invention relates to a fuel assembly to be loaded in a boiling water reactor, and particularly to a fuel assembly aimed at attainment of high burnup.
In a boiling water reactor, recently, high burnup for increasing the amount of energy generated per unit weight of fuel is expected from the viewpoint of both effective utilization of uranium resource and reduction in generation amount of spent fuel. To attain high burnup, it is necessary to enhance an average enrichment of a fissile contained in a fuel assembly. It is also known that, to promote effective reaction of a fissile for keeping stability of a boiling water reactor, it is required to increase a rate of the amount of a moderator with increased enrichment of a fissionable material.
A technique for effectively increase a rate of the amount of a moderator is disclosed FIG. 24 of in Japanese Patent Laid-open No. Hei 5-232273. This technique describes a fuel assembly 1 shown in FIG. 2, which has large-sized water rods 5, and short-length fuel rods 3 shorter than other fuel rods 2. In the fuel assembly shown in FIG. 2, the fuel rods are arranged in a square grid patten of 9-columns/9-rows, and two pieces of the large-sized water rods 5 are arranged in a region containing seven grid points at a central portion of the fuel assembly. The number of long-length fuel rods 2 is 66 pieces and the number of the short-length fuel rods is 8 pieces. The use of the short-sized fuel rods extends a flow passage of coolant on an upper portion of the fuel assembly, to increase the amount of a neutron moderator on an upper portion of a core, thereby promoting a fission reaction. The use of the short-sized fuel rods has another effect of reducing a pressure loss from the viewpoint of the flow of coolant, thus improving stability of the core.
A critical power of a fuel assembly is important as the scale of a thermal margin of the fuel assembly to be loaded in a boiling water reactor. The critical power is defined as a maximum thermal power of the fuel assembly within a range in which there occurs no boiling transition of fuel rods under a condition of the amount of coolant flowing in the fuel assembly. The larger the critical power of the fuel assembly, the larger the thermal margin. In many cases, since boiling transition occurs at a fuel rod exhibiting the maximum thermal power to restrict the critical power of the fuel assembly. Accordingly, the critical power becomes larger as a difference in power between the fuel rods becomes smaller and a power distribution in the fuel assembly becomes flatter. On the other hand, even when the power of the fuel rods is uniform, if there exists unevenness in the flow rate of coolant around each fuel rod, boiling transition tends to occur at a position where the flow rate of coolant is small, thus making small the critical power.
With respect to a thermal margin of a fuel assembly including short-sized fuel rods, Japanese Patent Laid-open No. Hei 5-232273 describes a technique in which short-sized fuel rods are provided at positions other than in the vicinity of corners of a fuel assembly, to flatten a power distribution in the fuel assembly. Besides, Japanese Patent Laid-open No. Hei 5-341071 describes the fact that upper portions of fuel rods adjacent to short-sized fuel rods are effectively cooled because a flow passage of coolant is wide on the upper side of upper ends of the short-sized fuel rods.
With respect to the fuel assembly having short-sized fuel rods described in Japanese Patent Laid-open No. Hei 5-232273, a configuration in which fuel rods containing burnable poison are arranged for reducing a power peaking at a cool state is disclosed in Japanese Patent Laid-open No. Hei 8-292281.
Of the above-described prior art fuel assemblies, the fuel assembly disclosed in Japanese Patent Laid-open No. Hei 5-232273 describes that the arrangement of the short-sized fuel rods at positions other than in the vicinity of the corners improves stability while flattening the power distribution in the fuel assembly; however, the document fails to sufficiently examine the effect on the flow of coolant.
In the fuel assembly disclosed in Japanese Patent Laid-open No. Hei 5-232273, the arrangement of the short-sized fuel rods at positions other than in the corner regions acts to increase the critical power from the viewpoint of flattening of the power distribution. On the other hand, according to the fuel assembly disclosed in Japanese Patent Laid-open No. Hei 5-341071, the arrangement of the short-sized fuel rods at positions other than in the corner regions acts to decrease the critical power from the viewpoint of flow of a coolant. As a result, the net effect of improving the critical power is unclear.
Japanese Patent Laid-open No. Hei 5-341071 describes the effect due to the local flow of coolant at positions of the short-sized fuel rods. The document, however, does not examine an effect on the flow distribution of the entire fuel assembly including positions apart from the short-sized fuel rods and an effect of the short-sized fuel rods on the power distribution in the fuel assembly.
Japanese Patent Laid-open No. Hei 5-232273 does not examine distributions of a fissile or burnable poison. On the other hand, Japanese Patent Laid-open No. Hei 8-292281 describes a distribution of burnable poison. The document, however, is intended to suppress a power peaking at a cool temperature, and it does not examine improvement in thermal margin upon operation of the feel assembly.